+1 on the 1yr answer. And I've also had a plastic surgeon tell me that the rays of the sun can penatrate clothing. Thus, sunblock is still recommended under the fabric.

You could also try sunblock underneath a water proof bandage of some sort. Just so you keep it from the adhering edges of the bandage. Some of the best sticking water prooof bandaids I've found are the kids/decorative ones called Tatoos... http://www.drugstore.com/nexcare-ban...-vary/qxp75863